Backflow preventer apparatus are well known in the art and are used in water distribution systems to prevent contamination of the potable water portion by preventing backflow of liquid from the non-potable portion beyond a certain point in the system. Two conditions tend to produce backflow in a water distribution system. The first condition which could cause backflow is known as "back siphonage" and this occurs in the potable supply pipe or main when its pressure drops to cause a vacuum or partial vacuum in that portion of the system. This will cause a backflow of liquid from the service pipe and if the liquid in the service pipe is contaminated, it will also contaminate the potable water supply. The second condition to cause backflow is a condition occurring in the service pipe or nonpotable portion of the system. This condition results in a back pressure condition when the pressure in the nonpotable service pipe exceeds that in the potable supply pipe.
The backflow preventer apparatus is usually installed with a gate valve on either side of the same in between a main or supply pipe for potable water and a service line which may be subjected to contaminants and thus be nonpotable water. Installations can be made at inlets to factories such as chemical plants and the like, or inlets to institutions, hotels or any large building complex.
In prior backflow preventer systems, vacuum breakers have been utilized upstream of the upstream check valve in a two check valve system or downstream of the downstream check valve in a two check valve system. When vacuum breakers were used upstream of the upstream check valve, they were provided primarily to take care of a backsiphonage condition where negative pressure occurred in the supply pipe. Such positioning of the vacuum breaker could be ineffective when the upstream or first check valve is fouled as the backsiphonage condition will permit water from the intermediate zone to pass through the first check valve with excessive check valve leakage. In the situation where the vacuum breaker was positioned downstream of the second or downstream check valve, its purpose was also to prevent backflow due to a backsiphonage condition. However, such positioning of the vacuum breaker required that it be higher than any portion of the service line. Otherwise, the pressure of fluid in the service line would maintain the vacuum breaker closed during the backsiphonage condition. Consequently, if the check valves were slightly fouled, backflow from the service line into the supply line would result.
Poppet-type check valves have been used in the past in backflow preventer apparatus, but they have not been entirely satisfactory because during a flow condition through the apparatus, there was too great a pressure loss especially at a high flow rate. The guides for the poppet-type check valve restricted the flow of liquid therethrough or caused turbulence. Additionally, the previous backflow preventer apparatus utilizing two poppettype check valves had a large cavity between the seats of the check valves and, thus, caused loss during flow conditions rather than having a substantially straight through flow line.